When food is present in the alimentary canal, cells in the gut secrete a hormonal signal (an “incretin”), which sensitizes the pancreas to the presence of glucose and results in a potentiated glucose-dependent insulin secretory response. Such a synergistic response to provide glucose-dependent insulin release (Kieffer T J and Habener, J R (1999) Endocr. Rev. 20, 876-913) is seen for the incretin signals, Glucagon-like Peptide 1 (GLP1) and Glucose-dependent Insulinotropic Peptide (GIP). These incretin signals typically exhibit short duration of action in the body, with GLP1 exhibiting a t1/2 of approximately 1-2 minutes (Knudsen, L B 2004, J. Med. Chem. 47, 4128-34). GLP1 and GIP are cleaved by an amino peptidase, dipeptidyl peptidase IV (DPPIV) and thus, the naturally occurring native hormone is not generally used in medicinal formulations. A peptide found in the saliva of the Gila Monster (exendin 4, Exenatide; Amylin Pharmaceuticals, San Diego, Calif.) was shown to bind to the GLP1 receptor and exhibit potent agonistic activity, thereby imparting a desirable glucose-dependent insulin secretory response (Nielsen L L, Young, A A, Parkes, D G (2004) Regul. Peptides, 117, 77-88). Exenatide and analogs of GLP1 have been administered to patients in need of treatment for type 2 diabetes.
Pituitary Adenylate Cyclase-Activating Peptide (PACAP) is a neuromodulatory peptide which stimulates PAC1, VPAC1, and VPAC2 receptors, and is emitted from nerve endings in the pancreas. Receptors of this general class reside in multiple tissues in the body, including in the pancreas. In the pancreas, stimulation of the VPAC2 receptors have been shown to provide a potentiated, glucose-dependent insulin release in response to elevated blood glucose levels similar to that of GLP1 or exenatide (Tsutsumi, M., et al. (2002) Diabetes 51, 1453-60). Thus such a stimulus (from PACAP or VPAC agonistic analogs) could be synergistic or alternative to incretin-like signals in stimulating glucose-dependent insulin release, since a similar profile of potentiated insulin secretion results from activation of a second class of receptor. Such an effect would be beneficial in the treatment of metabolic disorders, including Type 2 Diabetes Mellitus (T2DM), metabolic acidosis, insulin resistance and obesity. However, the naturally occurring native sequence of PACAP and its analogs also are typically short-lived in the body.
Synthetic exendin-4 is a relatively short acting peptide and there is a medical need for longer-acting peptides that can modulate glucose-dependent insulin secretion.